Project Summary/Abstract Norovirus is the leading cause of severe childhood diarrhea around the world and a major cause of acute gastroenteritis in all age groups. There are no currently approved vaccines or targeted therapeutics for norovirus infection and very little is known about the pathogenic mechanisms underlying gastroenteritis symptoms. To gain further understanding of this important virus, murine norovirus has been used as a model system for many years and has led to many significant advances in understanding norovirus biology. However, the absence of symptoms in immunocompetent adult mice infected with murine norovirus limits the applicability of this model to delineation of viral mechanisms of disease. We recently discovered that genetically wild-type neonatal mice develop acute, self-resolving diarrhea when infected with murine norovirus, a disease course that mirrors human norovirus infection. This novel small animal model of norovirus disease represents a major advance in the norovirus field since it will enable a complete characterization of viral disease mechanisms and ultimately serve as a platform to test the efficacy of antiviral compounds in vivo. While developing this new model, we observed differences in disease severity between genetically similar murine norovirus strains enabling identification of viral determinants of norovirus-induced disease. The objectives of this proposal are to elucidate the cellular tropism and pathogenic mechanisms underlying norovirus diarrhea using our novel symptomatic small animal model. In Specific Aim 1, I will test the hypothesis that infection of lymphocytes is critical to norovirus- induced diarrhea. This hypothesis is strongly supported by our in vitro findings revealing that, although virulent and attenuated strains replicate comparably in macrophages, only the virulent strain replicates in lymphocytes. Moreover, mutations abolishing lymphocyte infection in vitro are attenuating in adult interferon-deficient mice. In Specific Aim 2, I will test the hypothesis that virus-induced proinflammatory cytokines cause disruptions in tight junctions maintaining the intestinal epithelial barrier, consequently leading to increased intestinal permeability and diarrhea. This hypothesis is based on our observation that a virulent murine norovirus strain induces significantly more proinflammatory cytokine expression than attenuated strains despite comparable levels of viral replication; and on the well-established ability of proinflammatory cytokines to disrupt tight junctions. Overall, the studies described in this proposal will test our model that norovirus infection of intestinal immune cells induces a host response that is immunopathologic and leads to diarrhea via effects on the intestinal epithelial barrier.